One of the best tips I heard on being happy is 'don't believe everything you think'. The mind has a tendency to put a negative sheen over everything and tell you you're not good enough, not confident enough, not capable, did something badly etc., but when you step back and actually analyse these thoughts rationally, you suddenly realise that they're simply not realistic! Often you can beat yourself up over events or mistakes which are actually perfectly reasonable. For example, I've been training in a new job, and I went to a training event where in one task, I felt I didn't do very well. So my mind instantly told me I must be rubbish at the job, I shouldn't even be trying, I'm incapable of this kind of work, etc. But eventually, when I stepped back from this thought and analysed the event in question, I realised that the mistakes I made were only to be expected! I'm new to the job, I'm still learning, I'm bound to make mistakes, the person I was working with was being utterly unreasonable, and I must allow myself to make mistakes. That's a part of learning and growing. So in summary, don't believe everything you think; your mind is not the centre of wisdom and truth, but can often be too much of a perfectionist for its own good.
Secondly, and this is closely tied with the first one, be compassionate and gentle with yourself. If I had a friend who started speaking to me like my mind sometimes does (you're not good enough, you're not funny enough, you're incapable etc.), I would think they were completely horrible and totally out of order! A decent friend would tell you that you are a great person, that everyone makes mistakes, and would list all your good qualities (e.g. you're organised, polite, friendly, creative, kind, thoughtful, a hard worker etc.). Allow your mind to do this without guilt. I found it hard at first because my mind would attack me for being arrogant and for praising myself undeservedly, but I would simply argue with it - I do deserve this praise because these things are true about me. (I'm not crazy, I swear. :P) So in short, be nice to yourself; you love yourself really, under all that (often unreasonable) criticism.
Thirdly, a key part of happiness is positive thinking and gratitude. I recently started an exercise where, before sleeping each night, I would write down 5 things I was grateful for that day (even little things, like sunny weather, going for a walk in the woods, having time to relax and play video games, spending time with my friends or family etc.). It's surprising how this can affect the way you think during the day. I started thinking much more positively. If it was raining, I wouldn't think, 'Oh god well isn't this miserable', I'd think, 'This is actually kind of refreshing, and it's nice being inside in the warm'. So doing this exercise is quite effective to increase general happiness and I'd definitely recommend gratitude as a useful tool for positive thinking.
Compassion is another key part of happiness. Being compassionate and nice to others can have positive effects for you. Whenever I enter any kind of conflict with others, it dampens my mood for the rest of the day - especially if it turned out I was in the wrong or was too harsh on the other person. So with every interaction, remember to be kind and compassionate and to focus on making the other person feel appreciated and happy, even if they're, for instance, just one customer in a long line of customers at your till at work, or an acquaintance you don't know very well. Not only does it make the other person feel happy and valued, but it also increases your own happiness as you end each interaction with a good relationship with the other person, feeling well-liked and positive.
Finally, avoiding constant blame is another piece of advice I've found useful. You want the world to be a certain way - you want everyone to drive safely, you want people to be friendly to you, you want to do well in every exam you take, etc. - but often it is not this way. Rather than immediately blaming everything and everyone around you and casting a negative light on everything, just accept that these things do happen, and they are a part of life. For instance, if a car overtakes you and comes perilously close to scraping your car, you might want to get furious and rant about all the unsafe drivers out there, and how the world would be such a better place if these people didn't exist. But the fact is, they do exist, and ranting about them won't harm them, but only your own mood. So of course you can't really help feeling that flare of anger at first, but from there, think rationally, and don't let such an event get to you; the other person in question will probably have given it no thought at all!
Wow, sorry for the essay, but I've read some useful books on this subject and it's all coming back to me! I'd recommend 'How to be Happy' by Cara Stein who puts all this much better than I have. Hope you feel happier soon. :P